A transmission for a vehicle provides a speed-torque conversion (commonly known as “gear reduction” or “speed reduction”) from a higher speed motor to a slower but more forceful output. A continuous variable transmission (CVT), which uses a belt as a variator that is a transmission element, receives power transmitted from an engine by a primary pulley, transmits the power to a secondary pulley by the belt, and outputs the power, on which speed change is performed by the secondary pulley, to driving wheels.
In the above-mentioned CVT, since a speed ratio is continuously changed due to the difference in diameter formed between the belt and the pulleys, hydraulic pressure applied to a pulley piston of a movable pulley of each pulley is controlled. Accordingly, the diameters formed between the belt and the pulleys are changed, so that a desired speed ratio is obtained.
A pulley clamping force, which is a force of a pulley pushing the belt, should be appropriately formed in order to allow power to be smoothly transmitted between the pulleys and belt without slip. The pulley clamping force is determined depending on the input torque input from the engine to the pulley. In order to form the pulley clamping force determined as described above, target hydraulic pressure corresponding to the pulley clamping force must be appropriately determined, and the hydraulic pressure applied to the pulley must be controlled to be the target hydraulic pressure.
Meanwhile, the start condition of a vehicle is a driving condition that is sensitive to a driver even though a speed ratio is slightly changed. In this state, the CVT should maintain a full low speed ratio, which is the lowermost speed ratio of the CVT, stable.
That is, when a vehicle provided with a CVT starts, the CVT should be controlled so that a target speed ratio is stably maintained at the full low speed ratio, in order to prevent irregular change of vehicle speed and engine speed, thereby ensuring ensure a stable ride.
However, in the related art, at the above-mentioned start condition of the vehicle, a target speed ratio is constant at the full low speed ratio, but the change of the input torque from the engine is relatively large. Accordingly, the target hydraulic pressure applied to the pulley is changed due to the change of the input torque. If the target hydraulic pressure is changed as described above, the speed ratio is substantially changed. For this reason, there is a problem in that a driver has a discomfort ride.
The above information disclosed in this Background Art section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.